Polyethylenes are categorized according to their densities, which are may be used as a guide to end-use applications. For example, high density polyethylene (HDPE) has a low degree of branching, which results in a compact structure having high tensile strength. HDPE is used in products such as pipes and drums. Medium density polyethylene (MDPE) has a high degree of chemical resistance as well as shock and drop resistance, and may be used in products such as shrink films. Low density polyethylene (LDPE) possesses random long chain branching, with “branches on branches.” LDPE can provide good resistance to high temperatures and impact, and has been used in applications such as cling films and squeezable bottles. Linear low density polyethylene (LLDPE) has an essentially linear structure but also has low density because of its short chain branching, and is used in applications such as stretch films and coatings for cables.
Various processes can be used to produce polyethylene. In ethylene slurry polymerization processes, diluents such as hexane are used to dissolve the ethylene monomer, comonomers and hydrogen, and the monomer(s) are polymerized with a catalyst. Following polymerization, the polymer product formed is present as a slurry suspended in the liquid medium. In typical multi-reactor cascade processes such as those disclosed, e.g., in WO 2012/028591 A1, U.S. Pat. No. 6,204,345 B1, and WO 2005/077992 A1, monomer(s), hydrogen, catalyst and diluent are fed into the first of three reactors where a slurry forms from the polymer particles contained within the diluent and unreacted monomer. The reactors can be operated in parallel or in series, and the types/amounts of monomer and conditions can be varied in each reactor to produce a variety of polyethylene materials, including unimodal (molecular weight distribution) or multimodal polyethylene material. Such multimodal compositions are used in a variety of applications; e.g., WO 2012/069400 A1 discloses trimodal polyethylene compositions for blow moldings.
Ziegler type catalysts have been used in ethylene polymerization processes. These catalysts may use aluminum alkyl compounds as co-catalyst activators to activate titanium or vanadium sites on the catalyst. The amount of co-catalyst present in the reactor may determine the yields and selectivities of the ethylene slurry polymerization process, e.g. in multi-reactor systems, where different polymers can be produced in each reactor, but the same co-catalyst flows to each reactor in turn.
Various compounds, such as oxygen-containing polar molecules, can poison Ziegler type catalysts, degrading their yields and selectivities, as described, for example, in WO 95/07941 A1, WO 96/39450 A1, WO 2004/085488 A2, and EP 0 206 794 A1. This can occur, e.g., when the poisons interact with either the TiCl4 or the MgCl2 support of the catalyst. Aluminum alkyls such as triethylaluminum, trimethylaluminum, tri-isobutylaluminum and tri-n-hexylaluminum can be used as scavengers of poisons in polymerization solutions, as described in WO 2004/085488 A2. However, these aluminum alkyl materials are also co-catalysts for the catalyzed polymerization, as described above, so that the process of scavenging catalyst poisons changes the availability of the aluminum alkyl co-catalyst for ethylene polymerization.
Conventional ethylene polymerization processes may target an Al/Ti ratio in the reactor because the content of the aluminum alkyl co-catalyst may affect the activity of the catalyst and the properties of the produced polyethylene. However, when the content of oxygen-containing poisons in the feed varies, the effective Al/Ti ratio also changes because the level of active aluminum is reduced as the aluminum alkyl co-catalyst reacts with the oxygen-containing poisons. This reaction results in altered reactor yields and product properties. Moreover, polymerization plants periodically change catalysts to produce grades of polyethylene targeted for different end-use applications, e.g., switching from an injection molding grade to a film grade. Such catalysts may have different sensitivities toward poisons and toward changing levels of effective aluminum alkyl co-catalyst(s). Even more demanding is the situation where multiple slurry reactors are operated in series, where an active catalyst within the polyethylene product flows from reactor to reactor, and ethylene is fed to each reactor, but fresh aluminum alkyl co-catalyst is fed only to the first reactor. Therefore, a continuing need exists for ethylene polymerization processes that minimize the adverse effects on reactor yields and selectivities of changing feed contaminants.